Like almost all Rust projects, the entire point of the application-services components is that they be used by external projects. If these components use Rust features available in only the very latest Rust version, this will cause problems for projects which aren't always able to be on that latest version.
Given application-services is currently developed and maintained by Mozilla staff, it should be no surprise that an important consideration is mozilla-central (aka, the main Firefox repository).
It should also come as no surprise that the Rust policy for mozilla-central is somewhat flexible. There is an official Rust Update Policy Document but everything in the future is documented as "estimated".
Ultimately though, that page defines 2 Rust versions - "Uses" and "Requires", and our policy revolves around these.
To discover the current, actual "Uses" version, there is a Meta bug on Bugzilla that keeps track of the latest versions as they are upgraded.
To discover the current, actual "Requires" version, see searchfox
Our official Rust version policy is:
All components will ship using, have all tests passing, and have clippy emit no warnings, with the same version mozilla-central currently "uses".
All components must be capable of building (although not necessarily with all tests passing nor without clippy errors or other warnings) with the same version mozilla-central currently "requires".
This policy only applies to the "major" and "minor" versions - a different patch level is still considered compliant with this policy.
All CI for this project will try and pin itself to this same version. At
time of writing, this means that our circle CI integration
will specify the versions (and where possible, the CI configuration file will
avoid duplicating the information in
We should maintain CI to ensure we still build with the "Requires" version.
As versions inside mozilla-central change, we will bump these versions accordingly. While newer versions of Rust can be expected to work correctly with our existing code, it's likely that clippy will complain in various ways with the new version. Thus, a PR to bump the minimum version is likely to also require a PR to make changes which keep clippy happy.
In the interests of avoiding redundant information which will inevitably become stale, the circleci and rust-toolchain configuration links above should be considered the canonical source of truth for the currently supported official Rust version.